This is not a Back to My Mac (BTMM) issue since BTMM is not used for connections over a local network (LAN). This is a simple File Sharing issue. I suspect that if your are able to connect to the iMac from the MacBook Air but not the other way around that File Sharing is enabled on the iMac but not on the MacBook Air.
Make sure File Sharing is enabled on both computers. To do that go to the System Preference, under the Apple menu, and click on the Sharing button (looks like a folder). Now, put a check in the box next to File Sharing to turn it on.
Now open a Finder window. You should see the other Mac listed under the SHARED category on the sidebar. Select it and you should be presented with a list of available directories on the remote computer. Which directories you see and what permissions you have will be determined by whether you connect as a guest or using a user account on the remote computer. If you have an Apple ID setup on the both computers, in the Users & Groups account set up preference pane, then you can also connect using an Apple ID.